Kawasaki sold the Z200 single in the late 1970s.
The Kawasaki Z250 came in three models around late 1970s early 1980s:
Z250A twin with disk brakes
Z250B twin with drum brakes
Z250C single with drum brakes

Kawasaki sold a BR250 single in the late 1980s.

Yamaha had the SR175 and SR250 singles, both were a bit cruiser style.

Honda sold the CR250RS from about 1980 onwards, then the CBX250 with much the same motor but very different styling. The motor was essentially the XL250S motor. The CB250T and N models, also known as Superhawks in some places were twins, as were the CB200 and CB175 of the mid to late 1970s.

As I recall, the CB250RS, as you said, had pretty much the same motor as the XL250 but the CBX had a DOHC version of the engine.

I've never actually seen a CBX250 in the flesh as they weren't a great seller. Did see one advertised cheap earlier this year but circumstances conspired against my buying it. Suspect a DOHC 4 valve honda single could be quite a quick wee thing.

They also had plenty if 175-250cc street-only twins:
1968 CB175 (not in the US)
1969 to 1973 CB175
1974 to 1976 CB200
250 Nighhawk (1987-2008)
CB250 1970's, smaller CC version of both the CB350 and CB360 -- outside the US
1988-1990 VTR250 (I have a 1990)
200?-present VTR250 (not in the US)

One could also argue that the 1968-1973 CL175 and 1974-1976 CL200 were street only, as the "Scramblers" were really just CB175 and CB200 street bikes with a high-mounted scrambler-styled exhaust (and other minor styling differences).

They also had plenty if 175-250cc street-only twins:
1968 CB175 (not in the US)
1969 to 1973 CB175
1974 to 1976 CB200
250 Nighhawk (1987-2008)
CB250 1970's, smaller CC version of both the CB350 and CB360 -- outside the US
1988-1990 VTR250 (I have a 1990)
200?-present VTR250 (not in the US)

One could also argue that the 1968-1973 CL175 and 1974-1976 CL200 were street only, as the "Scramblers" were really just CB175 and CB200 street bikes with a high-mounted scrambler-styled exhaust (and other minor styling differences).

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those are all twins. there's a bazillion little 4 stroke twins. the small japanese street singles there are much fewer - especially above 100cc